1. Field of the Invention
The present invention relates to a radio frequency power amplifier used in cellular phone handset, and more particularly to a voltage power supply method of reducing mutual interference between circuits through a power supply line.
2. Description of the Prior Art
Recently, there has been an increasing demand for cellular phone handset as typified by cellular phones to be more compact in size and lighter in weight, and therefore research and development is being vigorously made to satisfy the demand. Conventional power amplifying circuits for transmission used in cellular phone handset require negative voltage power supply or negative voltage power creation circuit, have many part configurations, and cannot therefore meet the demand of being compact in size and light in weight. Accordingly, heterojunction bipolar transistors of Gallium Arsenide compound semiconductor (hereinafter simply referred to as GaAsHBT) are expected as amplifying devices used in radio frequency power amplifiers for transmission because they are capable of single positive voltage power supply and have excellent radio frequency characteristics.
FIG. 12 shows a conventional radio frequency power amplifier employing GaAsHBT, disclosed in Japanese Patent Laid-Open No. H10-75130. The radio frequency power amplifier comprises: an input impedance matching circuit 401; a power amplifying transistor 410; an output impedance matching circuit 402; and a base bias voltage control circuit 403 for the power amplifying transistor 410. The base bias voltage control circuit 403 comprises a transistor 411 and resistors 420 and 421. The reference numeral 430 denotes power supply for driving the collectors of the transistors 410 and 411, and the reference numeral 431 denotes power supply for controlling the gain of the radio frequency power amplifier, which is applied to the base of the power amplifying transistor through the base bias voltage control circuit. The base bias voltage control circuit, by substantially supplying a base current Ibb of the power amplifying transistor from the power supply for collector driving, is generally used to reduce a current Iapc for gain control to be supplied from the power supply for gain control and thereby reduce current supply capacity demanded to external control circuits to generate a voltage for gain control.
With saturation amplifiers complying with the GSM (Global System for Mobile Communication) system widely used principally in Europe presently, a trade-off relationship between output power and power added efficiency is a major problem in the development of radio frequency power amplifiers.
Moreover, with linear amplifiers complying with the W-CDMA (Wideband Code Division Multiple Access) system, which is one of third generation mobile communication systems, in addition to the trade-off between output power and power added efficiency, a trade-off relationship exists between distortion and power added efficiency, as described in page 36 in “Electronic Technology June, 2000” published by Nikkan Kogyo Shinbun.
Therefore, in the linear amplifiers, reduction in distortion leads to an increase in power added efficiency of radio frequency power amplifiers, which, in turn, leads to an increase in the performance of the radio frequency power amplifiers.